In the market of the cellular telephones in the GSM scheme as a de-facto standard, there is an increasing, indispensable need for those using the multi-band amplifiers, such as dual bands, in order for expanding the service area.
FIG. 1 is a schematic block connection diagram showing a configuration example of a conventional dual-band amplifier described in JP-A-2000-124829. The signal radio wave in a frequency band f1 is received by an f1-band input terminal 1000 and then removed of the interfering waves in the other band than f1 by a f1-band BPF (band-pass filter) 1002. Then, the signal is amplified to a desired level by an f1-band low-noise amplifier 1004 and inputted to one input terminal of a radio-frequency change-over switch 1006. On the other hand, the signal radio wave in a frequency band f2 is received by an f2-band input terminal 1001 and then removed of interfering waves in the other band than f2 by a f2-band BPF 1003. The signal is amplified to a desired level by an f2-band low-noise amplifier 1005 and inputted to the other input terminal of the radio-frequency change-over switch 1006. The radio-frequency change-over switch 1006 selects either one of the inputted f1-band or f2-band signal. The selected f1-band or f2-band signal is orthogonally demodulated by an orthogonal demodulating section 1007. In this manner, the orthogonal demodulating section 1007 is shared between the two frequency bands. In the case there exist three or more frequency bands, the orthogonal demodulating section 1007 is shared by a change-over selection at the radio-frequency change-over switch 1006.
In the configuration sharing an orthogonal demodulating section by using the radio-frequency change-over switch 1006, two of the low-noise amplifiers 1004, 1005 are required independently. Furthermore, loss is possibly caused by the radio-frequency change-over switch 1006.